High lead solders are high temperature solders that are often used to keep the internal connections of an electrical device properly positioned and stationed on a printing wire board (PWB). However, when the electronic devices containing lead solders are disposed, the lead from the lead solders still remains, which is considered to be hazardous to the environment and human health. As a result, stricter regulations increasingly prohibit the use of Pb-bearing solders in the electronic interconnection and the electronic packaging industries. Thus, Pb-free solders are widely being investigated as an alternative substitute to replace the traditional eutectic Pb—Sn solders. For example, SnAg, SnCu, SnAgCu and SnZn solders are now becoming the mainstream Pb—Sn alternatives for use within the semiconductor and electronics industries. However, the development of high temperature Pb-free solders that are appropriate for substituting conventional lead-rich (or the so-called high lead) ones, such as Pb-5Sn & Pb-5Sn-2.5Ag, are still in its infancy.
A common application of high temperature solders is often utilized to achieve die-attach, which is also known as die-bonding. In an exemplary process, an assembly is formed by soldering a silicon die onto a lead-frame using the high temperature solder. Then, the silicon die/lead-frame assembly, which may or may not be encapsulated, is attached onto the PWB by soldering or mechanical fastening. The PWB may then be further exposed to a few more reflow processes for surface mounting with other electronic devices. During the entire process, the internal connections between the silicon die and the lead-frame should be well maintained. To do so, this requires that the high temperature solder resist the multiple reflows without any functional failure. As such, the major requirements for high temperature solders include: (i) a sufficiently high melting temperature around 260° C. and above (in accordance with typical solder reflow profiles), (ii) the ability to achieve sufficient thermal fatigue resistance, (iii) high thermal/electric conductivity, and (iv) low manufacturing costs.
Currently, there are no drop-in lead-free solder alternatives that are available for high temperature solder use. While Bi—Ag alloys have a solidus temperature of 262° C. and thus satisfies the high melting temperature requirement for die-attach solders, there are a few major concerns with the use of such Bi—Ag alloys. For example, Bi—Ag alloys characteristically experience poor wetting on various surface finishes due to its weak bonding interface. As a result, Bi—Ag alloys are a poor choice for solder joint applications.
Additionally, while Sn-based alloys have good reaction chemistry with commonly used surface finish materials such as Cu/Ag/Ni/Au, alloying Sn into Bi—Ag is not ideal because it often leads to poor results. For example, alloying Sn into Bi—Ag will either result in poor wetting when insufficient amounts of Sn are alloyed, or result in a low melting point temperature of the Bi—Sn alloys when excess amounts of Sn are alloyed. This is largely due to the fact that Sn in the Bi—Ag alloy will form Ag3Sn in the intermetallic phases. During reflow, Ag3Sn will then have to dissolve back to the molten Bi matrix to release the free Sn. Thus, the free Sn will diffuse to the interface and react with surface finish materials for wetting. However, in most cases, the de-wetting of the molten Bi matrix occurs earlier than the occurrence of the free Sn being released, which leads to poor wetting capabilities. On the other hand, if there are too many Sn being alloyed, the excess Sn will form the low melting Bi—Sn phases, which again, also leads to an undesirable result since the low melting Bi—Sn will cause the material to lose its high temperature performance capabilities.